The invention finds use in the digital data receivers of cable modem termination systems (hereafter CMTS) of DOCSIS enabled cable television distribution systems. The DOCSIS 1.0 systems used time division multiplexed (hereafter TDMA) bursts only. However, the desire for higher speeds for transmission of digital data led to the development of DOCSIS 1.1 systems which were also TDMA systems but faster.
Upstream noise is a major problem in any system where digital data is transmitted upstream over a cable television hybrid fiber coaxial (hereafter HFC) cable distribution system to a cable modem termination system (hereafter CMTS). Privacy can also be a problem with TDMA bursts. One way of overcoming these noise problems and insuring privacy is to use code division multiplexing (hereafter CDMA) for upstream bursts. The code gain of CDMA systems helps overcome the noise and the spread spectrum nature of the signal prevents evesdropping by those without access to the spreading codes used in the transmitters. Terayon Communication Systems, Inc. of Santa Clara, Calif. has been a leader in bringing spread spectrum cable modems to the market. Terayon""s modems enjoy a further noise advantage because they use sychronous code division multiplexing (hereafter SCDMA) to cut down on intersymbol interference. SCDMA requires all remote modems to perform a ranging process to determine a delay which is proper for their distance from the CMTS such that frames of spread spectrum data transmitted from the remote modems all arrive at the CMTS with their frame boundaries aligned in time. This minimizes intersymbol interference caused by transmissions from other modems.
DOCSIS cable modem termination system receivers are under development by Terayon which are capable of receiving both TDMA and SCDMA bursts on different channels as well as both TDMA and SCDMA bursts in separate time intervals (with different MAP messages applying to each) on the same frequency channel. DOCSIS defines a sub-channels as bursts with frequencies and symbol rates such that there is overlapping bandwidth. Sub-channel bursts cannot be transmitted with overlap in time. Different type bursts with different multiplexing and/or different symbol rates can be transmitted during different time intervals on the same carrier, and because there is overlapping bandwidth, each interval would be a sub-channel. The term sub-channel, as it is used herein means bursts transmitted on the same or different frequencies with symbol rates and center frequencies of the RF carrier on which they are transmitted such that there is an overlap in the bandwidth, but multiplexed in time such that there is no overlap in time. Channels or frequency channel, as the terms are used herein means transmissions on carriers of different frequencies and at symbol rates such that there is no overlap in bandwidth. Because there is no overlap in bandwidth, transmissions on different channels may overlap in time.
A prior art receiver that could not receive two sub-channels or channels at different center frequencies referred to as the Jasper I was developed by the assignee of the present invention and is currently on sale. A United States patent application Ser. No. 09/792,815 filed by the assignee of the present invention on Feb. 23, 2001 describes circuitry of Jasper I, and that patent application is hereby incorporated by reference. The receiver of the above identified patent application is capable of receiving 15 different SCDMA and TDMA burst types by adjusting the operation of its circuitry using burst parameter data that define the burst to be received. The differences in the various burst types relate to the symbol rate, type of multiplexing, type of modulation, function of the burst such as initial ranging or periodic station maintenance or data, etc.
When deploying new cable modems (hereafter CMs) capable of higher speed TDMA transmissions and high speed SCDMA bursts into a system with existing slower DOCSIS 1.0 or DOCSIS 1.1 modems, there arises a backward compatibility problem. Cable operators devote a certain portion of the bandwidth of the HFC to upstream digital data transmissions and there is no other available bandwidth upon which the higher speed TDMA or SCDMA upstream bursts can be transmitted. The bandwidth of a channel is related to its symbol rate. The symbol rate of DOCSIS 1.0 and 1.1 modems is slower (1.28 or 2.56 Msps) than the new advanced PHY TDMA and SCDMA modems (5.12 Msps), so the new modem channels have wider bandwidth when transmitting at the faster symbol rates. Thus, it is frequently necessary for wide-bandwidth, high-speed TDMA and SCDMA advanced PHY channels to overlap in bandwidth with lower-speed, more narrow bandwidth channels on which the older DOCSIS 1.0 and 1.1 modems transmit. This is because of the bandwidth and frequency band limit restrictions on upstream transmissions imposed on digital data delivery services by the cable operators.
The Jasper I receiver chip cannot receive mixed mode signals, i.e., two different sub-channels at different symbol rates and/or different multiplexing types which have overlapping bandwidth and the same center frequency. Further, it could not receive two different sub-channels having overlapping bandwidth and different center frequencies. The Jasper I receiver also cannot receive different channels at different RF center frequencies. Further, the Jasper I receiver was capable of receiving bursts at a maximum of 5.12 Msps, so when it was receiving DOCSIS 1.0 or 1.1 bursts at 1.28 or 2.56 Msps, the digital circuitry was idle most of the time.
Therefore, there is a need for a receiver that can receive bursts on multiple different RF inputs to keep the shared back end digital circuitry busy all the time. Further, there is a need for a CMTS receiver that can receive, on each RF input, mixed mode transmissions, that is having multiple sub-channels with overlapping bandwidth and either the same or different center frequencies. The receiver should be able to receive sub-channels of any type on the same frequency channel such as DOCSIS 1.0, Advanced PHY TDMA or advanced PHY SCDMA, and the different sub-channels may have different symbol rates. The receiver should be able to receive different sub-channels having overlapping bandwidth and the same center frequency or different center frequencies. Further, the receiver should be able to receive, multiple different channels without overlapping bandwidth and having the same or different symbol rates and/or multiplexing types and different center frequencies which are spread far enough apart given the symbol rate that there is no overlap in bandwidth. For example, there is a need for a receiver that can receive a single advanced PHY SCDMA channel having a symbol rate of 5.12 megasymbols per second (Msps) and a center frequency of F1 transmitted with its bandwidth overlapping the bandwidths of multiple separate other channels of DOCSIS 1.0 bursts having symbol rates of 1.28 Msps or 2.56 Msps and center frequencies on both sides of F1 and spaced apart so that the DOCSIS 1.0 channel bandwidths do not overlap each other. The receiver must be able to receive a different UCD message for each sub-channel type and switch between sub-channels on the fly during a guardtime between bursts on different sub-channels or different channels.
Channels with overlapping bandwidth and different burst types are typically multiplexed in time so that two different burst types with overlapping bandwidth are not transmitted at the same time, but there is also a need for a receiver that can simultaneously receive two or more sub-channels with overlapping bandwidth and overlapping in time. To receive overlapping bandwidth bursts of different types which are time division multiplexed, the circuitry of the needed CMTS receiver has to be adjusted using burst parameter data that defines the burst to be received during any particular time on a sub-channel or channel.
The Jasper I receiver could only receive one channel of RF signals at a time, i.e., it only had one RF input which could only be coupled to one physical transmission medium. Thus, if a headend cable modem termination system (CMTS) were coupled to four different HFC systems, four different Jasper I CMTS receivers would have to be used with attendant multiplication of space consumed and cost. Frequently, CMTS headend apparatus are coupled to multiple HFC systems, each serving different groups of customers in different areas. Typically, each CMTS system has a mixture of older DOCSIS 1.0 and 1.1 type cable modems (CM) and newer advanced PHY TDMA and SCDMA CMs. Further, rack space and floor space is limited so footprint size of the CMTS equipment is an important consideration. Typically, cable operators have to buy the CMTS receiver equipment as well as thousands of CMs to serve their customers that want broadband digital data services delivered over their cable TV system. Thus, expense of the CMTS system is an important factor to commercial success because costs will be passed along to customers.
Thus, there is also a need for a CMTS receiver which has multiple RF inputs for connection to multiple HFC systems and which is capable of receiving the different burst types which can be transmitted in a DOCSIS system with a mixed bag of DOCSIS 1.0 and 1.1 CMs plus advanced PHY ATDMA and SCDMA CMs. Such a receiver must be able to receive upstream digital data transmissions at each of these multiple RF inputs which have different multiplexing and/or different symbol rates. Further, the circuitry coupled to each RF input must be able to receive different sub-channels on the same frequency channel with some bursts on some sub-channels being DOCSIS 1.0 or 1.1 TDMA and other bursts on different sub-channels being advanced PHY TDMA or SCDMA (referred to herein as mixed mode) with different symbol rates. Such a receiver must also be frequency agile on each of its RF inputs so as to be able to receive different frequency channels having different center frequencies without overlapping bandwidth.
The genus of the invention is defined by a central receiver for a distributed system of digital data transceivers which have the following characteristics that provide multichannel, mixed mode reception capability:
a plurality of channel receivers, each capable of receiving mixed mode upstream bursts in different sub-channels that have overlapping bandwidth but which are multiplexed in time, where each subchannel burst may have a different symbol rate, different RF frequency, different multiplexing type and different Synchronous Code Division Multiple Access (hereafter SCDMA) frame size;
a shared back end circuit for recovering the data from each burst, making measurements and calculations on at least some bursts transmitted by each cable modem which are sent down to the cable modem which sent said burst which are useful in establishing at least frame boundary and minislot boundary synchronization and upstream equalization;
an arbiter coupled to receive the data output by each said channel receiver and structured to supply received data to said shared back end circuit such that said shared back end circuit is shared so as to process all data from all said channel receivers at different times;
control circuitry for controlling at least said plurality of channel receivers and said shared back end circuit to provide multichannel, mixed-mode reception of digital data.
xe2x80x9cMultichannelxe2x80x9d refers to the capability to receive multiple channels simultaneously on different channel receivers. If the different channel receivers are coupled to different distributed systems such as different Hybrid Fiber Coaxial cable (HFC) distribution systems, the different channel can have overlapping bandwidth. However, if the different channel receivers are coupled to the same HFC system, the different channels being received simultaneously cannot have overlapping bandwidth. xe2x80x9cMixed-modexe2x80x9d refers to the reception of different types of bursts on the same general channel, i.e., with overlapping bandwidth, but multiplexed in time. In other words, a single RF carrier is divided into an interval during which a burst of a first symbol rate and first multiplexing type is modulated on the carrier, and one or more other intervals when bursts having different symbol rates or multiplexing types are modulated onto the carrier. xe2x80x9cMixed-modexe2x80x9d also refers to the overlapping in bandwidth but not overlapping in time transmission of multiple subchannels on different carriers having different center frequencies which are not so different as to preclude overlapping bandwidth.
Although multiple channels can be received, and each channel can be mixed-mode, the total throughput through the backend circuit is limited to that circuit""s capabilities, which, in the preferred embodiment, is 5.12 megasymbols per second. The advantage of the multiple front end receivers is that the backend circuit can be fully utilized as compared to the prior art wherein a single front end receiver circuit fed the back end circuit. Thus, if the burst being received had a symbol rate of less than 5.12 megasymbols per second, the full capability of the backend circuit was not used.
The analog front end circuits do wide band sampling. The digital front end receivers are controlled to connect to the right analog front end circuit, mix the samples of each subchannel down to baseband using the correct local oscillator signal frequency. The samples are then resampled at the proper symbol rate, decimated down to a predetermined number of samples per symbol, typically two, and narrow band noise is excised. A shared back end demodulator detects impulse noise and marks symbols corrupted therewith with erasure bits, despreads the spectrum of SCDMA bursts, recovers the symbol clock and makes start of burst measurements in support of ranging. The preamble of each burst is processed to develop phase and amplitude error correction factors and upstream equalization coefficients. The data symbols are then decoded in the appropriate type of decoder such as a Viterbi decoder for TCM bursts and the Reed-Solomon code words are reassembled and error corrected and the payload data is output. The erasure bits written by the impulse detector are used to prevent corrupted symbols from being used by the tracking loops for symbol clock recovery or error correction factor development or equalization tap coefficient updating. A SOVA Viterbi decoder in the preferred embodiment uses erasure indications to control the branch metric values in the decoding process, and outputs erasure indications which are used to increase the range of error detection and correction of the Reed-Solomon decoding circuitry.